http://www.viperalley.com/ubbthreads.phpWell, I took some time to look into this and was unable to come up with a universal solution; however, I did add Sub-Forums to my site (intelligent ones that indicate new posts - and in the case of the Anything Goes area, an indicator of the number of new posts); however, my solution is specific to my site as the database layout is not subforum friendly.
There are two tables: the categories table and the boards table. Using this layout, there isn't an obvious way to make roomfor sub-forums without adding some kind of indentifier to the boards table. In doing so, you could potentially build upon the $forum array built in ubbthreads.php (when you encounter a subcat flag, add to the Description string for the last cateogry added to the array, for example).
I didn't want to spend a week on this, so did it my own way but hopefully someone has some time to invest in getting a better solution.